Iron ore pellets are spherical agglomerates of iron ore super-fines (below 0.25 mm). Pellets undergo thermal processing to increase their compressive strength, which is important during handling and shipping, and during their use in reduction reactors. During thermal treatment several physico-chemical reactions occur, leading to strong changes in microstructure. These changes can be described by “degrees of maturation” (DM), and can be classified in classes in order to represent the microstructural characteristics. Typically DM is divided into four levels: A, B, C, and D, although intermediate types, such as AB, BC, and CD can also be obtained. The Degrees of Maturation increase from A to D, considering class A as the lowest Degree of Maturation and class D as the highest Degree of Maturation.
Usually, the DM is defined by observation of a cross section of the sample under the optical microscope. A trained human operator observes the images, either directly on the microscope eyepiece, or on a digital display, and decides the DM for that field of view. This decision is based on several parameters such as particle shape, porosity, presence of ferrite and slag formation. This represents a complex classification problem that has a strong subjective component, leading to errors and lack of reproducibility. FIGS. 1 to 4 show typical images for classes A, B, C, and D.